This invention relates generally to hydraulic valve actuators, and more particularly to a hydraulic valve actuator and an electro-hydraulic system for controlling the operation thereof.
Various valve actuators of the pneumatic and hydraulic type are known in the art. One hydraulic valve actuator is described, for example, U.S. Pat. No. 3,394,632. However, in prior art pneumatic and hydraulic actuators, it has generally been necessary to provide an external source of pressurized hydraulic fluid or pressurized air, as the case may be, and connecting lines leading from the source to the actuator. Owing to space limitations in many installations, this external source must often be located remotely from the actuator and valve, with relatively long connecting lines provided therebetween. In many cases, the valve to be controlled is part of a system in which extremes of temperature are present. These temperatures may cause undesirable effects on the fluid or air in the connecting tubes, or even cause damage or rupture of the tubes themselves. Further, such systems generally are entirely manually operated, a mechanical indicator of valve position being mounted on the valve shaft, for example, while the valve controls are generally mounted in a location remote from the actuator and valve. In such prior art systems, therefore, it is often difficult for an operator to simultaneously monitor the valve position and operate the valve controls. Moreover, there is no provision in such systems for retaining the valve in a desired position, once such a position is established, and therefore, the valve and its condition must be frequently or even continuously monitored by an operator.
Another prior art valve control system utilizes an electric motor actuator connected with the valve shaft. But when relatively large valves are to be actuated, very large electric motors and corresponding high voltage power lines are required, thus adding appreciably to the expense of such a system with respect to the relatively simpler hydraulic systems of the above-cited patents. Such electric motor actuators are shown, for example, in U.S. Pat. Nos. 3,430,916 and 3,687,415.
It will be appreciated that an all-electric system may be readily adapted for use with modern types of electronic automatic control sytems, thus eliminating the problems of manual operation associated with the hydraulic actuators referred to above. Several such control systems are described, for example, in the following publications:
Bulletin E-262, Remote Indicator and Control of Actuator Travel, published by the Hills-McCanna Company; Bulletin E-261a, Modular Solid-State Servo Amplifier, published by the Hills-McCanna Company; Product Data sheet PDC-1, March 1976, Modular Solid-State Process Controller, published by the Hills-McCanna Company; Bulletin No. A-3B, Ramcon Rotary Motion Control, published by the Hills-McCanna Company.
Some electric motor type actuators cannot be provided with fail-safe operating mechanisms. Broadly speaking, fail-safe devices are connected to the actuator so as to return the valve to a predetermined position, generally either the fully opened or fully closed position, when power to the actuator is lost for any reason. In contrast, such a fail-safe system has been used with pneumatic valve actuators, and generally comprises a resiliently biased spring member adapted to return the actuator to the predetermined "fail-safe" position upon loss of pneumatic pressure, as the case may be. Such a system is illustrated, for example, in the above-cited Bulletin No. A-3B.